1. Field of the Invention
The invention is generally related to colloidal silica sols, and more particularly, to functionalized colloidal silica sols as a ceramics additive.
2. Description of the Prior Art
Through the hydrolysis of tetraalkyl ortho-silicates, spherical, monodisperse particles of colloidal silica can be prepared. The surface of these particles is modified to stabilize them in water or organic solvents. Surface modified collodal silica particles are referred to as functionalized, as are the resulting collodal solutions, or sols.
Surface modification of colloidal silica particles has been generally limited to binding polymer chains through surface esterification of surface silanols or through a urethane linkage. For example, aminopropyl, octadecyl, mercaptopropyl and polyethylene oxide groups have been bound to colloidal silica particles in order to stabilize the colloidal silica sols in water. In organic solvents, polystyryl anions, trimethoxysilyl-terminated polymers, and monohydroxy-terminated poly(dimethylsiloxane) have been reacted with the silica surface to provide stable colloidal silica dispersions.
Ceramics used in high-tech applications are generally prepared from a fine powder which is shaped and fired to achieve a desired microstructure or properties. The most popular method for shaping high-tech ceramics is dry-pressing. Dry-pressing may be defined as the simultaneous uniaxial compaction and shaping of a fine granular ceramic powder with small amounts of water and/or organic binders during confined compression in a die. The popularity of dry-pressing is attributable to its inherent ability to rapidly form a wide variety of ceramic pieces having a variety of shapes, while maintaining close tolerances and controlled compact character.
Spray-drying is a process which produces fine ceramic powder used in dry-pressing. Aluminas, zirconias, titanates and ferrites are examples of ceramic powders produced by the spray-drying process. Generally, with regard to spray-drying, a slurry containing one of the above minerals is transformed into a dry powder by spraying the slurry into a hot drying gas. The majority of spray-dried ceramic powders are generated from water-based slurries. In more detail, the slurry is pumped through an atomizer located in the drying chamber. The slurry is broken down by the atomizer into a large number of uniform, spherical droplets. These droplets are spherical-shaped because of the effects of surface tension. Water is promptly evaporated from the droplets and the resulting dry powder is stored for future use in dry-pressing and firing operations. Properly done, spray-drying is an economical and continuous operation which produces a fine powder of uniform character.
Slurries used in spray-drying typically include dispersants and binders. Dispersants are organic polymers adsorbed onto the surface of the particles by Coulombic or van der Waals forces or hydrogen bonding. A wide variety of dispersants is available and the properties of the more common dispersant are well known in the ceramic art. Binders are added to powders to increase the green strength of the compacted, non-fired article. Selection of the binder system is based on cost, purity, ease of thermal removal, compaction behavior/green strength, and sensitivity to environmental conditions (i.e., ambient humidity). The binder can be a water-soluble type, such as poly(vinyl alcohol), or an emulsion of an organic substance dispersed in water. Probably the most common binder used in spray-drying ceramic powders is poly(vinyl alcohol). Dispersants and binders are eliminated from the green (non-fired) ceramic piece during a sintering (firing) step, wherein the additives are "burnt-out". Depending on the temperature and time required to burn these additives out, imperfection can occur in the finished ceramic piece.